Ricin is a very toxic protein obtained from the castor bean seeds. Ricin is a glycoprotein with a molecular weight of about 64-65 kD composed of an A chain having a weight of about 31 kD and a B chain having a molecular weight of about 33 kD. The A chain functions to inactivate ribosome and inhibit protein synthesis of cells.
Each of the toxin's two subunits has a distinct role in toxicity. The B chain selectively binds to residual groups of galactose on the membrane surfaces of eukaryotic cells. The toxin is able to access the cytosol. It has been postulated that the toxin can enter the cytosol from an intracellular compartment not accessible to extracellular antibodies (Sandvig, et al ((J. Biol. Chem. 257, 7504-7513 (1982)).
The study of the ricin B subunit in translocation of the ricin into the cytosol has been studied fairly extensively. It has been proposed that intracellular galactose receptors mediate the effect of the ricin B. Antibodies to the ricin B chain have been shown to protect against the pathological effects of ricin toxin. U.S. Pat. No. 4,520,011 to Neville, et al. discloses and claims a method of inhibiting protein synthesis in target cells by addition of excess ricin B to compositions containing ricin A chain attached to antibodies that target particular cells. By adding an excess of ricin B chain it was possible to increase the effect of the targeting antibody/ricin A complex on the target cells. This increase in effect on target cells was attributed to the effect of ricin B in facilitating entry of ricin A into the cytosol of the targeted cell.
U.S. Pat. No. 4,490,362 to Shionoya, et al. teaches use of the B chain of ricin as a generalized immunopotentiator to increase immune response to infectious diseases. That reference does not suggest use of the B chain alone as a vaccine against ricin toxin.
Compositions containing either the A chain of ricin or the B chain of ricin are available commercially. Recombinant A chain has been made by recombinant methods. (See, for example, U.S. Pat. No. 4,689,401 to Ferris, which is incorporated herein by reference in its entirety.) The use of the A chain in medicinal science wherein the A chain is coupled with a targeting moiety to target to particular cells as a means of selectively destroying malignant cells is known. The ricin A chain has, in some instances, been modified when used in a conjugate for purposes of selective destruction. See, for example, U.S. Pat. No. 4,980,457 to Jansen and Gros and U.S. Pat. No. 4,962,188 to Frankel. However, the use of the ricin A chain in a vaccine to provide immunogenic activity to elicit antibody response for protection against the pathological effects of exposure to the complete ricin has not been known.
A first generation ricin toxoid vaccine composed of ricin toxin that has been treated with formaldehyde has been developed and is in pre-clinical evaluation. While the efficacy of this vaccine appears to be adequate, safety concerns center on three issues: 1) Highly lethal toxin is used as a starting material; 2) potential for reversion exists; and (3) involvement of formaldehyde in conjunction with heat in the treatment and stabilization of the vaccine preparation can lead to unacceptable side effects and a rather variable product.
The exact sequence of the ricin toxin A chain (RTA) can vary in nature from one variety of castor bean seed to another.